An immersion oil which increases numerical aperture of objective lens is sometimes used for enhancement of magnifying power of microscopy. As the immersion oil, there have been known those which are mainly composed of glycerin, silicone oil or polybiphenyl chloride (PCB).
However, glycerin has hygroscopicity and a lower refractive index and silicone oil also has the defects of high viscosity and low refractive index. On the other hand, PCB is harmful for human bodies.
In order to solve these problems, an immersion oil composed of diphenylmethane derivative and liquid paraffin has been proposed in Japanese Patent Examined Publication No.35053/80.
Furthermore, U.S. Pat. No. 4465621 discloses an immersion oil for microscopy comprising butylbenzyl phthalate and chlorinated paraffin.
These immersion oils nearly suffice various properties required for immersion oils for microscopy such as refractive index, Abbe's number, viscosity and resolving power, but still have the problem of too high fluorescene according to measurement by spectrophotometer.
That is, fluorescence microscopy uses as a light source an ultra-high pressure mercury lamp which emits ultraviolet rays which excite fluorescence. Exciting lights used in this case include U excitation which comprises excitation of fluorescence by a filter of mainly 365 nm in wavelength, V excitation which comprises excitation by bright line ultraviolet rays of 405 nm or 435 nm or by wide exciting light of VB excitation, B excitation which comprises excitation of fluorescence by an exciting filter of mainly 410 nm and G excitation which comprises excitation of fluorescence by an exciting filter of mainly 546 nm. Since fluorescence emitted from a sample by these exciting lights is observed by fluorescence microscopy, immersion oil to be used is desirably less in emission of fluorescence. The immersion oil mainly composed of a compound having a plurality of aromatic rings such as disclosed in Japanese Patent Examined Publication No.35053/80 and U.S. Pat. No. 4465621 emit much fluorescence and cannot effectively be used for fluorescence microscopy. The immersion oil disclosed in U.S. Pat. No. 4465621 has further problem of inferior hue.